In a report assessing whether production wells at a TVA power plant could threaten local drinking water, investigators found that a shallow aquifer containing arsenic, lead and other contaminants is connected to the vital Memphis Sand aquifer.

During tests in which water was pumped from the wells at the Tennessee Valley Authority's Allen Combined Cycle Plant, there was a "discernible drawdown" in the shallower Alluvial aquifer even though the wells drew from the Memphis Sand, TVA said Wednesday in summarizing a remedial investigation report submitted to the Tennessee Department of Environment and Conservation.

The drawdown, according to the summary, means there's a "hydraulic connection" between the Alluvial aquifer, where high levels of arsenic and other toxins were discovered near a TVA coal ash pond, and the Memphis Sand, the source of tap water throughout Shelby County.

However, in releasing the findings, TVA emphasized that the investigation also found no evidence that arsenic or other contaminants had seeped into the Memphis Sand. Agency officials further said the studies indicate the flow of groundwater in the shallow aquifer is generally horizontal, not downward into the Memphis Sand.

Still, spokesman Scott Brooks reaffirmed TVA's commitment to cool the plant with water purchased from the Memphis Light, Gas and Water Division — and not pumped from the plant wells — for at least the time being. As a contingency measure, TVA is building two 2.5 million-gallon storage tanks at the plant to hold water bought from MLGW, he said.

The remedial investigation, involving TVA, the U.S. Geological Survey and the University of Memphis, was launched last year after the federally run utility reported finding arsenic in concentrations of up to 400 times the drinking water standard in monitoring wells installed in the Alluvial aquifer near a coal ash pond at the nearby Allen Fossil Plant. Elevated levels of fluoride and lead also were detected.

The findings of the investigation could further stoke a long-running controversy over the five wells the agency installed at the Allen plant, which is set to begin operations late this spring.

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Environmentalists and researchers opposed TVA's plans to pump an average of 3.5 million gallons daily to cool the plant, saying the withdrawals could suck contaminants from the Alluvial into the Memphis Sand through suspected gaps in a protective clay layer separating the aquifers.

Scott Schoefernacker, program manager for the Center for Applied Earth Science and Engineering Research at U of M, said the question of whether the TVA pumping would contaminate the Memphis Sand remains unanswered.

"All that our tests show is that there's a connection," he said.

Although they had failed to block local permits authorizing the TVA wells, opponents said Wednesday that the report vindicates their arguments.

The findings also point to the need for stricter regulations to protect groundwater, he said.

Amanda Garcia, staff attorney for the Southern Environmental Law Center, said in a prepared statement that the investigation "confirms our biggest fear — that there is a hydraulic connection between the arsenic-contaminated groundwater and the Memphis Sand aquifer, the city’s drinking water source."

Debate over the issue dates back to 2016, when TVA had just begun work on the $975 million Allen plant. The facility will replace the coal-fired Allen Fossil Plant, which is being retired as part of an agreement to resolve air pollution violations.

TVA dropped its initial plans to use treated effluent from a nearby sewage treatment plant to cool the new Allen facility. The agency said it instead would tap the Memphis Sand because the highly complex and efficient plant needs water that's of extremely high quality. TVA officials ruled out using the less-pure Alluvial aquifer, and they said MLGW couldn't supply adequate amounts of water needed for cooling.

TVA applied for permits under Shelby County's well construction ordinance to drill five 650-foot-deep wells into the Memphis Sand. The county's Groundwater Quality Control Board granted approval in 2016, with members saying they had no grounds on which to deny the permits because the application met all the requirements of the ordinance. But critics said the regulations were inadequate and didn't provide for suitable public notice.

In response to the TVA project, the groundwater board has proposed an overhaul of the ordinance. The revised rules would require a 30-day public notice period and hearing for large-scale wells and documentation of a "justifiable need" to tap local aquifers. For wells used to obtain cooling water, the rules stipulate that strict and costly "best available control technology" be used to conserve water.

The rules will be the focus of public hearings set for 5:30 p.m. March 19 at the Benjamin J. Hooks Central Library and 2 p.m. March 20 in the code enforcement training room at 6465 Mullins Station Road.

Reach Tom Charlier at thomas.charlier@commercialappeal.com or 901-529-2572 and on Twitter at @thomasrcharlier.